In telecommunications and network applications, transmitters and receivers are interconnected by a signal transmission medium, such as a coaxial cable or an optical fiber. Faults in the signal transmission medium often cause undesired signal attenuation and even loss of signals transmitted over the medium. Time-domain reflectometry is a well-known technique for testing signal transmission media to determine the location and severity of such faults.
Time-domain reflectometry is functionally similar to radar. Pulses are periodically launched into the transmission medium by a transmitter. During the intervals between transmitted pulses, a return signal is received that includes reflections indicative of faults in the medium. The elapsed time between launching a pulse and receiving a reflection is indicative of the distance to the fault.
In OTDR applications, pulses of light are periodically launched through an optical fiber. Light propagating through the optical fiber is scattered by the fiber material by a mechanism referred to as "Raleigh Scattering" that causes some of the light to return through the fiber to the transmitter. Such returned light is referred to as "backscatter." Backscatter signal amplitude decreases exponentially as a function of the distance along the fiber.
Light reflections from other causes are referred to as events. Events have various causes. A fiber discontinuity, such as a splice, causes a loss of light amplitude in the fiber, but no light reflection. Such an event appears as an anomalous drop of backscatter amplitude starting at a distance in the fiber corresponding to the splice location. A fiber discontinuity, such as a mechanical connector, causes a pulse reflection that is added to the backscatter. An irregularity in the fiber can both be reflective and have light loss.
Locating such events is useful for determining fiber quality and fault locations. An OTDR typically displays a plot of return signal amplitude versus time from which the locations and amplitudes of events can be determined relative to a reference event such as an end of the fiber or a known connector or splice location.
There are previously known apparatus and methods for detecting and characterizing irregularities in fiber optic cables. In particular, U.S. Pat. No. 5,155,439 issued Oct. 13, 1992 for a METHOD OF DETECTING AND CHARACTERIZING ANOMALIES IN A PROPAGATIVE MEDIUM, assigned to the assignee of this application, describes a method for locating and characterizing reflective and nonreflective events in a backscatter return signal. The method my be used in a prior art OTDR such as the model OF235 Fiber Optical Time-domain Reflectometer manufactured by the assignee of this application. Such an OTDR samples the return signal at closely spaced time intervals representing points spaced along the length of the fiber. The resulting sampled data points are displayed on the OTDR as an amplitude-versus-time waveform. Because backscatter is a relatively weak signal, each data point displayed in the waveform is repetitively sampled and averaged to reduce noise to an acceptable level. Depending on the fiber length and desired waveform display resolution, a considerable amount of time (perhaps 10 minutes) may be required to collect and display the waveform. Finally, a human operator must interpret the displayed waveform to determine the event locations and amplitudes.
In addition to backscatter and events, echoes present in the return signal are a serious source of waveform contamination. Echoes can appear as real events even when no such event actually exists at its indicated location. Echoes, therefore, cause confusion and can lead to misinterpretation of faults by the OTDR operator.
Echoes in OTDR waveforms are caused by reflections off fiber components such as connectors and mechanical splices. Echoes are specifically related to valid reflective events in the waveform. When the launch pulse encounters a discontinuity (such as a connector), part of the pulse is typically transmitted, and part of the pulse is reflected back to the OTDR where it appears as an event. Echoes occur when reflected pulses encounter other fiber discontinuities while returning to the OTDR. A portion of the reflected pulse continues toward the OTDR while the remainder is rereflected in the same direction as the launch pulse. If the rereflected pulse encounters yet another reflective event, a portion will again be reflected toward the OTDR where the echo could appear as an event.
FIG. 1 schematically shows the process by which echoes develop in an optical fiber. A distance in an optical fiber, as measured from an OTDR front panel, is represented by a distance axis 10, and time is represented by a time axis 12. A launch pulse 14 is represented by a solid line that originates at a point 16 and propagates through the fiber to a fiber end point 18. The slope of the line is n.sub.eff /C where n.sub.eff is the effective group index of the optical fiber, and c is the velocity of light.
Reflective events are represented by vertical lines including a front panel event 20, a fiber end event 22, and discontinuity events 24 and 26. When launch pulse 14 encounters a reflective event, a portion of the pulse is reflected, and a remaining portion is transmitted. For example, when launch pulse 14 encounters discontinuity event 26, a reflected portion 28 and a transmitted portion 30 divide at the distance and time represented by a point 32.
To be observed by the OTDR, a pulse must be reflected an odd number of times. Valid data are contained in pulses that are reflected only once. In contrast, echoes are pulses that are reflected three, five, or more odd times.
Echoes are classified by the number of reflections they have undergone. First-order echoes are pulses that have been reflected three times, second-order echoes are those that have been reflected five times, and so on. FIGS. 2 and 3 schematically show the development of typical first- and second-order echoes. The number of echoes grows dramatically as the echo order number increases. For example, FIG. 1 shows, as dashed lines, only the first-order echoes present in a fiber with just four reflective events. Fortunately, because of fiber attenuation, echoes beyond second order are usually not detectable by the OTDR. However, this generalization has exceptions such as multimode cable installations having many highly reflective events spaced closely together.
Echoed pulses cannon be absolutely separated from valid pulses because the OTDR is a noncoherent, onedimensional laser radar than measures only the amplitude and time of the returning pulses. A conventional OTDR display, therefore, intermixes echo data and valid event data, thereby leaving to the operator the difficult task of interpreting the cluttered display.
What is needed, therefore, is an OTDR that can identify which return pulses are valid reflections and which are echoes, and alert the OTDR operator accordingly.